An X-ray CT (Computed Tomography) apparatus is provided with an X-ray source for irradiating a subject with X-rays, and an X-ray detector for detecting X-rays passing through the subject, the X-ray detector being opposed to the X-ray source, and the X-ray CT apparatus reconstructs an image from variations of an X-ray attenuation factor inside the subject, by using a data processing system, on the basis of projection data in a plurality of directions, the projection data being obtained by rotational imaging around the subject. Typically, an X-ray tube is used as the X-ray source, which irradiates an electrode with electrons accelerated by high voltage, and generates X-rays by utilizing a bremsstrahlung process. In order to take an image of a wide range at once, the X-ray detector being frequently used, is provided with a two-dimensional array of X-ray detector elements, each made up of a combination of scintillator and photodiode.
Spatial resolution of an image obtained by the X-ray CT apparatus, depends on a focus size, the number of projection data items obtained per rotation, i.e., the number of views, and array density of the X-ray detector elements of the X-ray detector. If there is a need for improvement in any of those described above, a large amount of development costs are required.
Patent document 1 discloses FFS (Flying Focal Spot) technique, as a technique for enhancing spatial resolution of the X-ray CT apparatus. In the FFS technique, the focal spot is oscillated at high speed during rotational imaging, thereby doubling sampling density at the center of a scan field of view (SFOV).
In the FFS, the sampling density can be increased in the array direction (e.g., channel direction) of the sampling elements, which is a moving direction of the focal spot. However, the CT apparatus performs imaging by continuous rotation, and thus, it is not possible to obtain data items at two focal spots of the FFS, simultaneously at the same projection angle. Therefore, it is necessary to interpolate deficient data, in order to obtain data also in the view direction, with respect to each projection angle.
As one of data interpolation techniques in the X-ray CT apparatus, there is known an interpolation method that is used for rebinning data measured by a fan beam, so that the data is acquired just like measured by a parallel beam. It should be noted that conversion from the fan beam to the parallel beam is simply referred to as Fan-Para conversion.
The patent document 2 describes one of the Fan-Para conversion techniques, which performs interpolation in the direction along a trajectory on a sinogram, depicted by the center of a display field of view (DFOV).